Olefins, such as ethylene, may be polymerized by contacting them under polymerization conditions with a catalyst to produce a granular polymer. The granular polymers produced usually contain residual gaseous or liquid alkenes and alkanes as well as other hydrocarbons. These hydrocarbons should be removed from the granular resin for many reasons including, for example, quality control of the final end product and safety reasons. In addition, proper disposal of the hydrocarbon is required in order to meet environmental standards concerning hydrocarbon emissions.
There are various techniques for removing volatile hydrocarbons from polymers. For example, U.S. Pat. Nos. 4,197,399, 3,594,356, and 3,450,183 disclose a columnar (or straight cylindrical) vessel used as a purger, referred to as a polymer purge bin, or product purge bin. U.S. Pat. No. 4,372,758 discloses a degassing or purging process for removing hydrocarbons, such as alkenes, from solid olefin polymers. The purging process generally comprises conveying the solid polymer (e.g., in granular form) to a polymer purge bin and contacting the polymer in the purge bin with a countercurrent inert purge gas stream to strip away any hydrocarbon vapors that are released from the polymer. Nitrogen is most commonly used as the inert purge gas. However, it is also possible to use a light hydrocarbon rich gas to strip the heavier hydrocarbons in a first stage and then use an inert gas in a second stage for the comparatively easy task of stripping the light hydrocarbons that remain in and around the resin after the first stage.
A vent recovery system is typically utilized to recover hydrocarbons, such as an olefin monomer, from the mixed hydrocarbon and inert purge gas stream exiting the purge vessel. Existing methods of recovering hydrocarbons in the polymerization process vent gas include: (a) compression and condensation with at least one of water or air, mechanical refrigeration, and ethylene expansion, to cool to approximately −10° C.; and (b) separation via pressure swing absorption (PSA) or membranes. In existing gas phase polyethylene plants, option (a) is most commonly used, however, a combination of option (a) and option (b) may also be used.
In a compression and condensation system, such as described in U.S. Pat. No. 5,391,656, a polymer purge bin vent stream containing an inert gas, such as nitrogen, and an olefin monomer is treated in a series of steps that include: (a) cooling to condense a portion of the polymerization vent gas; (b) separating the condensed liquids from the remaining non-condensable light gas; (c) compressing the non-condensable light gas; (d) cooling the compressed stream to promote further condensing; (e) further separating the condensed liquids from the remaining non-condensable light gas; and (f) recycling the condensed liquids containing the olefin monomer.
Conventional compression and cooling vent recovery systems using ambient air or water cooling may recover most of the heavier hydrocarbons, such as butene, isopentane, hexene, hexane, and other heavy alkanes and olefins, contained in vent gas. However, the amount of hydrocarbon recovery is constrained by the practical limit on the ambient cooling medium supply temperature. Consequently, a conventional vent gas recovery system will typically recover only up to 50% of the vented ethylene monomer, causing loss of valuable hydrocarbon and increased flaring.
Furthermore, the inert gas, such as nitrogen, remaining in the polymerization vent gas after the condensed liquid separation, may still contain significant amounts of heavier hydrocarbons, precluding its re-use as a resin drying or purge gas. To reach a higher ethylene recovery and achieve a higher quality of recovered gas, further processing may be required.
Refrigeration systems, including mechanical refrigeration and olefin expansion, may also be used for cooling in polymerization vent gas separation. Refrigeration has certain advantages over conventional ambient cooling. For example, refrigeration systems may achieve a final condensation temperature of below 0° C., and thus may be more efficient in hydrocarbon removal from polymerization vent gas.
Mechanical refrigeration uses a compression refrigeration system to provide a coolant, such as chilled brine or glycol mix, to the vent recovery area. Mechanical refrigeration units (MRU) typically achieve a final polymerization vent gas condensation temperature as low as −10 to −20° C., thus facilitating additional liquid hydrocarbon recovery via condensation. However, MRU's require high equipment costs and unit operating costs associated with increased power usage and refrigerant handling. In addition, MRUs may require the introduction of new and potentially toxic chemicals to the site, such as halo-fluorocarbons, for compression refrigeration of brine or glycol, which may not be desirable.
Olefin expansion may also be used for vent gas recovery, wherein condensation of hydrocarbons in a polymerization vent gas containing non-condensable inerts, such as nitrogen, is accomplished via partial expansion of a high-pressure olefin. U.S. Pat. No. 5,391,656 discloses such a process of “free refrigeration,” where the ethylene is partially expanded from a high pressure, such as about 800-1000 psig, to a lower pressure required to supply the ethylene purification system upstream of the polymerization unit, such as a pressure of about 350-400 psig. Similar to the typical MRU operation, the partial expansion of ethylene may generally achieve a final condensation temperature of −10 to −20° C., sufficient to condense a high percentage of the ethylene monomer contained in the process vent gas. However, a significant amount of ethylene may still remain in the non-condensed vent gas.
Therefore, there still exists a need for an improved method and apparatus to separate hydrocarbons from polymerization vent gas that would: (a) recover and reuse more of the valuable olefin monomer; (b) reduce flaring of unrecovered hydrocarbons; and (c) allow re-use of vent gas containing inerts, such as nitrogen, as a purge medium for the polymer purge bin.